Process for improving the printability of the surface of alpha-cellulose paper to be used as the decorative sheet in a decorative laminate

ABSTRACT

A PROCESS FOR IMPROVING THE PRINTABILITY OF A-CELLULOSE PAPER WHICH IS TO BE USED AS THE DECORATIVE SHEET IN A DECORATIVE LAMINATE COMPRISING PLACING THEREON A DISCONTINUOUS COATING FROM AN ORGANIC SOLVENT SOLUTION OF A MIXTURE OF CERTAIN ACRYLIC COPOLYMERS WITH AN INORGANIC PIGMENT.

United States Patent 'ice 3,578,483 PROCESS FOR IMPROVING THE PRINTABILITY OF THE SURFACE OF a-CELLULOSE PAPER TO BE USED AS THE DECORATIVE SHEET IN A DECORATIVE LAMINATE Leonardo Medrano Garcia, Lawrence, Mass., assignor to Formica Corporation, Cincinnati, Ohio No Drawing. Filed May 14, 1969, Ser. No. 824,689 Int. Cl. B44d 5/02, 5/03 U.S. Cl. 11738 5 Claims ABSTRACT OF THE DISCLOSURE A process for improving the printability of (X-CBHHIOSB paper which is to be used as the decorative sheet in a decorative laminate comprising placing thereon a discontinuous coating from an organic solvent solution of a mixture of certain acrylic copolymers with an inorganic pigment.

BACKGROUND OF THE INVENTION Decorative laminates have been manufactured throughout the world for a substantial number of years and have been utilized for the manufacture of furniture panels and building panels such as the side walls of rooms. These decorative laminates are prepared by superimposing a plurality of core sheets, such as kraft paper sheets, impregnated with a phenolic thermosetting resin; upon one another which core sheets are surfaced with a decorative sheet, such as an tat-cellulose paper sheet, which is either a solid color sheet or a printed sheet. The decorative sheet is generally impregnated with a noble thermosetting resin, such as a melamine-formaldehyde resin. These OL-CCIIUIOS6 sheets are generally white in color but as they are manufactured they frequently have certain surface imperfections which impairs the print fidelity and color rendition of the tit-cellulose sheet. To overcome these imperfections in the a-cellulose sheet, the process of the present invention has been developed which improves the printability of the a-cellulose sheet to be used as the decorative sheet in such a decorative laminate and also does not impair impregnability of the sheet with the laminating resin.

FIELD OF THE INVENTION The present invention is in the field of treating a-cellulose paper sheets with a binder/pigment dispersion in an inert organic solvent so as to deposit on the surface of said sheet a discontinuous coating of a plurality of discrete deposits on said paper so as to smooth or level out the surface imperfections of the paper which permits the sub sequent rotogravure printing of patterns or designs and ultimately results in a significant improvement of print fidelity, clarity and color rendition of a decorative laminate produced therefrom.

DESCRIPTION OF THE PRIOR ART The instant application is not aware of any truly significant prior art.

SUMMARY OF THE INVENTION A process for improving the printability of u-cellulose paper to be used as the decorative sheet in a decorative laminate comprising placing a discontinuous coating of a plurality of discrete deposits on said paper of a coating 3,578,483 Patented May 11, 1971 composition comprising 1) a binder copolymer of (A) from about 50 to mol percent of an alkyl ester of an acrylic acid and correspondingly (B) from about 50 to 5 mole percent of an acrylic acid, (2) a lightcolored nonabrasive inorganic pigment having a particle size between about 0.1 and 5.0 microns dispersed in (3) an inert organic solvent and evaporating the solvent so as to deposit on said paper from about 0.1 to 1.0 grams/ft. of binder/ pigment in individual deposits, wherein the pigment to binder ratio is between about 1.5 :1 and 2.5: 1.

The coating composition used in the present invention will be described in greater detail hereinbelow. This coating composition may be applied to the paper by several different means. The application procedures must be capable of delivering the resinous formulation to the surface of the paper in such a manner that the printing surface is modified but a continuous coating is not developed. This can be achieved for example by the use of a standard rotogravure cylinder which is uniformly etched. A suitable type would be one that would have an average cell depth of about 40 to about 50 microns and has between about 133 and lines per inch. A sufficient amount of the coating composition should be applied to the paper so that upon the evaporation of the solvent therefrom, there will be deposited about 0.1 to about 1.0 gram per square foot and preferably 0.5 gram per square foot of the binder/pigment solids. After the application of the coating composition to the a-cellulose paper, a normal ink drying cycle such as about one minute at about 150 C. is applied so that the paper is adequately dried and is ready for printing in a conventional manner.

The acrylic polymeric material that constitutes the binder in the coating composition of the present invention is prepared by copolymerizing two essential polymerizable monomers. The first polymerizable monomer is an alkyl ester of an acrylic acid wherein the alkyl group is derived from a monohydric alcohol having from 1 to 4 carbon atoms; namely methanol, ethanol, propanol and butanol. The acrylic acid may be acrylic acid per se or methacrylic acid, ethacrylic acid, chloroacrylic acid, and the like. More particularly, one may use methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, and the like. These esters may be used singly or in combination with one another. The preferred ester is methyl methacrylate. The other essential comonomer used to prepare the copolymers are any of the acids referred to hereinabove in describing the esters. Similarly these acids may be used singly or in combination with one another. If desired, additional monomers may be used in preparing a terpolymer in which acrylamide, methacrylamide or the methylolated derivatives may be used or acrylates such as hydroxy ethyl acrylate, hydroxy methyl acrylate, hydroxy propyl acrylate and ethylaminoethyl acrylate and the like may be used. The copolymer should contain a preponderant amount of the alkyl ester of an acrylic acid such as between about 50 to about 95 mol percent of said alkyl ester; and correspondingly, from about 50 to 5 mol percent of an acrylic acid. The preferred acid to be used in the copolymer is methacrylic acid and the preferred mol percent ratio is about 90:10 ester to acid, respectively.

The pigment used in the coating composition of the present invention may be any light colored, non-abrasive inorganic pigment including titanium dioxide which gives the desired opacity to the base paper. One may use, for instance, su-ch pigments as precipitated calcium carbonate, barium sulfate, titanium dioxide, aluminum oxide, and the like. The most suitable pigment is kaolinite clay which is the preferred pigment. The particle size of the pigment is critical and should be selected for use in these paper coating formulations within a particle size range from about 0.1 to about 5.0 microns. In order to be applied on a paper substrate by the rotogravure coating technique, it is preferred to use a coating grade clay with an average particle size of about 0.2 micron. The applied coating is discontinuous in the sense that it only corrects topographic imperfections of the raw paper which results in a smoothing and leveling of the surface.

The solvent used to disperse the binder/pigment mixture can be any one of the readily available inert inorganic solvents such as benzene, toluene, xylene, ethyl Cellosolve, diacetone alcohol, ethylene chloride, trichloroethylene, butyl Cellosolve, acetone, methyl ethyl ketone, tetrahydrofuran, Cellosolve acetate and mixtures and the like. The amount of solvent used may be varied between about 30% and 70% by weight based on the total weight of the coating composition. The final ratio of solvent should be such that a formulation with optimum printing viscosity of 50-70 Kreb units, preferably 58-62 Kreb units.

As an optional modification, one may incorporate into the coating composition surface active agents such as the non-ionic surface agents of which a substantial plurality are available commercially, such as the octyl phenoxy ethanols, the nonyl phenoxy ethanols, and the like. These agents serve to give adequate Wetting of the pigment and flow properties of the formulation both in printing and contact with the paper. The desired range is 0.05 to 0.5% based on total weight. A particularly desirable non-ionic surface active agent which is commercially available is the water soluble benzyl ether of octyl phenol condensed with ethylene oxide.

'It is also advantageous, although not imperative, to

I make use of a pigment dispersant of which a plurality are COATING COMPOSITION A 80 parts of a 100% solids acrylic copolymer prepared by copolymerizing 90 mol percent of methyl methacrylate and mol percent of methacrylic acid are blended into a mixture of 100 parts of xylene and 60 parts of ethyl Cellosolve. The blend is heated to the reflux temperature until the polymer is completely dissolved in the solvent mix; whereupon the solution is cooled to room temperature. Into a stirred mixture of 150 parts of ethyl Cellosolve and 0.5 part of a commercially available non-ionic s rfactant (benzyl ether of phenol condensed with ethylene oxide) and 2.0 parts of a pigment dispersant (sodium salt of a polymeric carboxylic acid), there is added 145 parts of a commercially available kaolinite clay. The pigment slurry is added to the resin solution with mechanical stirring and the pigmented acrylic resin is then transferred to a pebble mill and allowed to grind for a period of 16-24 hours. The thus ground coating composition is then ready for use.

Example 1 A sheet of tit-cellulose paper is passed on contact with a rotogravure cylinder, carrying a coating of composition A, which is uniformly etched to an average depth of about 50 microns and about 150 lines per inch. The pick-up of the coating composition is 0.5 gram per square foot. The

4 thus coated paper is then heated for one minute at 150 C., whereupon the inert organic solvents are flashed oil and the paper is printed in a conventional manner.

The resultant wood design bearing sheet is impregnated with a commercially available melamine formaldehyde resin and dried. A laminating assembly was prepared by stacking, in a superimposed relationship, a melamine formaldeh de resin impregnated a-cellulose overlay sheet, the resin impregnated precoated print sheet described above and 8 sheets of a standard phenolic resin impregnated core stock. A polished press plate was placed next to the overlay sheet member and the entire assembly was then inserted in a laminating press and consolidated at a temperature ranging from C. to C. under a pressure of 1400 p.s.i. for 20 minutes. Following lamination, the entire assembly was cooled to room temperature and removed from the press.

The decorative surface of the resulting laminate possesses significantly improved print fidelity and color rendition than was previously possible. All other properties of the "laminate were tested and complied with the NEMA Standard LP 1-1964.

Example 2 Example 1 is repeated in all details except that the coating composition is derived from an acrylic copolymer containing 60 mol percent methyl methacrylate and 40 mol percent ethacrylic acid and a rotogravure cylinder is used which has cell depths of 40 microns and 133 lines per inch.

Example 3 Example 1 is repeated again using a coating composition derived from an acrylic copolymer containing 80 mol percent methyl methacrylate and 20 mol percent methacrylic acid with a solvent of 100 parts of toluene and 60 parts of Cellosolve acetate. This composition is applied by a rotogravure cylinder with cell depths of 50 microns and lines per inch.

Comparative Example 4 Example 1 is repeated in all details except that the coating composition A is applied by means of a knife drawn down giving a completely continuous coating. Improved print fidelity is obtained, however, as a result of inadequate melamine impregnation in the print, the ultimate laminate was deficient in heat resistance and the ink layer blistered in a boiling water test.

Comparative Example 5 Example 1 is repeated in all details except that polyvinyl alcohol is used as a binder rather than the acrylic resin. Printability and color rendition were comparable to that in Example 1 except that the laminate properties of heat resistance and resistance to boiling water were deficient because of inadequate melamine impregnation of the print sheet.

Comparative Example 6 Example 1 is repeated in all details except that the precoating of the print base stock is omitted. A laminate produced thereby was deficient in color rendition and hue.

I claim:

1. A process for improving the printability of a cellulose paper to be used as the decorative sheet in a decorative laminate comprising placing a discontinuous coating of a plurality of cells on said paper of a coating composition comprising (1) a binder copolymer of (A) from about 50 to 95 mol percent of an alkyl ester of an acrylic acid and correspondingly (B) from about 50 to 5 mol percent of an acrylic acid, (2) a light-colored non-abrasive inorganic pigment having a particle size between about 0.10 and 5.0 microns dispersed in (3) an inert organic solvent and evaporating the solvent so as to deposit on said paper from about 0.1 to 1.0 gram/ft. of binder/pigment in individual cells having a cell depth between about 40 and 50 microns 4. The process according to claim 1 in which the pigand between about 133 and 150 lines per inch, wherein ment is kaolinite clay.

the pigment to binder ratio is between about 1.5 :1 and 5. The process according to claim 1 in which the coat 2.5:1. ing is applied to the paper from a rotogravure cylinder.

2. The process according to claim 1 in which the binder 5 is a copolymer of methyl methacrylate and methacrylic References C'ted acid. UNITED STATES PATENTS 3. The process according to claim 2 in which the methyl 3 259 515 7 1966 Packer 117 38 methacrylate and methacrylic acid are present in a 90 n mol percentzlO mol percent ratio, respectively. 10 WILLIAM J. VAN BALEN, Primary Examiner 

